Lovers often carve their initials in the smooth gray bark of beech trees. Now those beloved trees—which can reach nearly 40 meters tall, live up to 400 years and are among the most abundant forest trees in the Northeast and Midwestern U.S.—are increasingly threatened by beech leaf disease.

In 2012, a Greater Cleveland naturalist noticed odd, dark, leathery stripes between some veins of a few beech leaves. Since then, beech leaf disease has spread faster and faster around the lower Great Lakes and the Northeast, ravaging one of the region’s most vital trees. 

In 2019, the disease was found in four states and Ontario. And by 2022, as both the disease and its detection rose, it spread to 12 states, plus Ontario and the District of Columbia.

“’22 was the wakeup call for any dismissiveness,” Robert Marra of the Connecticut Agricultural Experiment Station said. 

Little is known about the possible role of climate change. Dan Herms, vice president of research and development at the Davey Institute in Kent, Ohio, said the disease seems typical of invasive blights over the centuries. But Marra speculates that the nematodes, or roundworms, overcrowd leaves during dry spells and burst out after erratic downpours. Either way, the canopy’s decline adds more heat to already overheated areas.

The disease has struck all beech species, including the widespread American beech, endemic to eastern Canada and the eastern and central U.S. That species makes up about 25 percent of forest trees in Northeast Ohio. It also ranks as the third-most abundant forest tree in Connecticut and the most abundant in Washington, D.C., metro area parks.

Like other trees, beeches reduce pollution and floods. They also provide shelter, shade and nuts for many animals, including foxes, black bears, black-capped chickadees, blue jays, grouse and ducks. Their roots host symbiotic fungi, which in diseased trees are losing nutrition and often dying as fall nears, according to an April report in the Journal of Fungi by Holden Forests and Gardens outside Cleveland and Bartlett Tree Research Laboratories in Charlotte, North Carolina.

The disease has several allies, including the spotted lanternfly and the centuries-old beech bark disease. Still, a 2021 report showed leaf disease far surpassing bark disease. The former turned up in nearly half of the beeches studied around Lake Erie and the latter in fewer than 4 percent.

Beeches are among many kinds of trees that reproduce partly through their roots, especially when under stress. So beech saplings are proliferating, crowding out other species that might fare better over time.

Year by year, infected trees produce fewer, smaller, darker leaves, which photosynthesize less. Eventually, branches start to wither. Most saplings die within five years of infection and mature trees within 10, according to David Burke, Holden’s vice president of science and conservation. 

In 2021, a report in Phytobiomes Journal showed that infected leaves have high levels of a fungus and of four kinds of bacteria, raising suspicions that they might cause the disease. But most researchers think those microorganisms play no more than a secondary role and mainly prey on already stricken leaves.

The researchers mostly blame a nematode, or roundworm. The diseased leaves’ tell-tale stripes resemble ones caused by other nematodes in crops and flowering plants.

A beech bud can hold up to 18,000 of these microscopic, sinuous, sticky organisms, according to researcher Paulo Vieira of the U.S. Department of Agriculture in Beltsville, Maryland. They winter in the bud, then attack the emerging leaves. They travel between leaves when the surfaces are wet. They travel between trees with suspected help from birds, insects and breezes.

The same nematodes are native to Japan but do little harm there. Typically, pathogens native to one country can be more harmful in other geographies, where their prey haven’t built up resistance. The U.S. Forest Service plans to fund trips by four researchers to study Japan’s beeches in 2024 and 2025.

Amid the rapid spread of the disease, scientists are making progress in understanding and possibly mitigating it.

For six years, the Cleveland Metroparks and Northeast Ohio’s Davey Institute have been treating diseased beeches with phosphite. Davey’s Herms said that the treatments seem to reduce nematodes and symptoms in parks and yards. But no one’s about to treat a whole forest.

Emelie Swackhamer, an educator with the Penn State Extension, said of the blight, “I think it’s going to be pretty bad. To lose the environmental services of another key species is really upsetting.”

But Holden’s Burke sees signs of resistance. “We see a lot of trees suffering from BLD and some that look good.” He’s propagating the good ones and hoping that they’ll spread well in depleted forests.

“I don’t think they’re going the way of the American chestnut,” Burke said of the beeches. Instead, he thinks they may go the route of ash trees, which the emerald ash borer has sharply reduced but not wiped out.